In semiconductor fabrication processes, the resolution of a photoresist pattern begins to blur at about 45 nanometer (nm) half pitch. To continue to use fabrication equipment purchased for larger technology nodes, double exposure methods have been developed.
Double exposure involves forming patterns on a single layer of a substrate using two different masks in succession. As a result, a minimum line spacing in the combined pattern can be reduced while maintaining good resolution. In a method referred to as double dipole lithography (DDL), the patterns to be formed on the layer are decomposed and formed on a first mask having only horizontal lines, and on a second mask having only vertical lines. The first and second masks are said to have 1-dimensional (1-D) patterns, which can be printed with existing lithographic tools.
Another form of double exposure is referred to as double patterning technology (DPT). Unlike the 1-D approach of DDL, DPT in some cases allows a vertex (angle) to be formed of a vertical segment and a horizontal segment on the same mask. Thus, DPT generally allows for greater reduction in overall IC layout than DDL does. DPT is a layout splitting method analogous to a two coloring problem for layout splitting in graph theory. The layout polygon and critical space are similar to the vertex and edge of the graph respectively. Two adjacent vertices connected with an edge should be assigned different colors. Only two “color types” can be assigned. Each pattern on the layer is assigned a first or second “color”; the patterns of the first color are formed by a first mask, and the patterns of the second color are formed by a second mask. A graph is 2-colorable only if it contains no odd-cycle and loop. Although DPT has some advantages, it is more computationally intensive than DDL.
In some cases, even if a pattern is separated into vertical and horizontal components, one or more pairs of features on a single mask are too close to each other for exposure with high resolution. If two features are so close that a high resolution exposure cannot be obtained by DDL or DPT decomposition, the pattern is said to have a “native conflict”. Such a problem can be addressed by changing the layer design, possibly at great expense, or by increasing the spacing between devices, which increases the die area.
Improved methods for automating the double exposure decomposition process are desired.